It is known to scrub acid gases, especially carbon dioxide and hydrogen sulfide, from gas mixtures which can also include polymerizable organic components, with an aqueous scrubbing liquid in which a key component is an alkanolamine such as ethanolamine.
Such a system has been described in German Pat. No. 19 24 052.
In this process, the scrubbing liquid, following the scrubbing operation, is heated, is regenerated (to remove the carbon dioxide and hydrogen sulfide), is cooled and is returned to the scrubber.
The polymerizable components or polymers or polymerizates formed therefrom are removed from the scrubbing liquid with a water immiscible organic solvent which is used to extract the scrubbing liquid.
This process can be used to remove carbon dioxide and hydrogen sulfide from cracking gases which are produced by a partial oxidation or pyrolysis of hydrocarbon mixtures. While this publication describes washing liquids which can be solutions of potash, alkanolamines or aminoacid salts, best results were found in this system with aminocarbonic acid salts, aminosulfonic acid salts or aminocarboxylic acid salts which have a high absorption coefficient for the acid gases as well as a low pickup of hydrocarbons. These latter scrubbing agents are also easily regenerated.
A special problem, however, arises when the scrubbing process encounters in the raw gas resin-forming agents, especially dienes, higher trienes or acetylenes, unsaturated polycyclic and heterocyclic components and other polymerizable organic components which can also include sulfur-containing organic compounds.
These are carried with the scrubbing liquid into the regenerating apparatus and tend to react in the latter with heating to form polymers which are insoluble in the scrubbing liquid and deposit in the regenerating column, in the cooler downstream thereof and in the boiler which is provided in the sump of the regenerating column to heat the liquid phase.
To overcome this disadvantage it has been proposed in the prior process to remove the polymerizable components with the aid of a water immiscible organic solvent which is used to extract the scrubbing liquid.
For the extraction of the polymerizable components by the conventional process, there are three techniques which have been proposed.
Firstly, the extraction is carried out directly in conjunction with the scrubbing operation in an extraction column provided immediately downstream of the scrubbing column at the temperature at which scrubbing is effected. The extracting solution is thus contacted with the scrubbing liquid only in the extraction column at the scrubbing temperature prior to heating of the charged scrubbing liquid to the regeneration temperature. This has the disadvantage that the extraction effectiveness at the relatively low scrubbing temperature is much less than optimum.
A second technique which has been proposed is to carry out the extraction directly in the regenerating column. The pure extraction solvent is introduced into the head of the regenerating column and trickles downwardly in the direction of flow of the charged washing liquid through the packing of the regenerating column. A portion of the aqueous scrubbing liquid is heated at the bottom of the column to produce a vapor phase which rises through the packing. The principal disadvantage of this technique is that the regenerating column must handle not only all of the scrubbing liquid but also all of the organic solvent serving as the extracting medium. A portion of the extracting solvent, by contact with the rising hot vapors, is also converted into the vapor phase and must be cooled and condensed at the head of the regenerating column to be recovered.
For efficient operation, the regenerating column must be dimensioned significantly larger than would be the case for the processing only of the scrubbing liquid and the construction of the column is further complicated by the need for providing a phase-separating device at the lower portion of the column so that the organic solvent can be removed from the aqueous mixture.
The refluxing and condensing device at the head of the regenerating column must be designed to permit not only condensation of vapors of the scrubbing liquid but also vapors of the extracting solvent. Finally, operation of the system is complicated by the fact that one must deal with a two-phase mixture rather than a single gas-solubilizing liquid phase in the form of the scrubbing liquid.
In a third alternative, the organic solvent serving as the extracting agent is circulated together with the scrubbing liquid throughout the scrubbing-liquid circulating path. The clean extraction solvent is supplied together with the regenerated scrubbing liquid at the head of the scrubbing column and both liquids are withdrawn together from the sump of the scrubbing column, are heated, are passed through the regenerating column and are further cooled. Thereafter a portion of the liquid mixture is withdrawn from the circulation and separated in a phase separator into an aqueous scrubbing-liquid phase and an extraction-solvent phase containing the polymerizable components. The scrubbing liquid component is returned to the liquid mixture withdrawn from the sump of the regenerating column while the extraction solvent portion is, after removal of the polymerizable components, returned to the head of the scrubbing column.
In this process, apart from the obvious disadvantage that the regeneration of the scrubbing liquid is loaded with the full amount of the extracting solvent as described previously, the circulating system must be dimensioned to deal with all of the extracting solvent as well as with the scrubbing liquid. This increases the required pump capacities and the power consumption of the system.
Since the extracting solvent is not fully treated after picking up the polymerizable components, there is an incomplete removal of the latter components and as a result the volume rate of flow of the extracting solvent must be greater.